lighting is key
You will need to be sure the lighting and fertilization is good so the plants can take in the nitrate. 20 ppm is a bit high, but if you plant the tank first and get the plants running well, the plants will suck that down to zero and then you can add back the new water and adjust the total nitrates that way. That is, do a 50% water change with the tank at 0 ppm and add 50% with 20 ppm and you have 10 ppm overall, perfect! With good lighting and correct phosphates and traces that 10 ppm will need a boost in just a few days.
I am not certain that you can do this with only 15% changed every other day, I think you will need to severely limit the number of discus you keep with a water change schedule like that. Now, the rule of thumb is 1 discus per 10 to 15 gallons, but if you are not going to change water a lot, then you need to back off that number. So, rather than keeping something like 10 or 12 discus, if you keep 6 or 8, then you can handle the lower water changes, but I'd still suggest you shoot for 30% 3x/week.
You will also need to keep a close eye on the pH and KH, for KH 45ppm is ~2 degrees of hardness which puts you in danger of a pH crash due to normal metobolic processes in the tank (fish respiration and bacterial processes) You can use crushed coral (slow acting) or baking soda (fast acting) to add KH, you'll just need to monitor this a lot, though the discus can take it quite acidic they won't like sudden changes, I'm not sure about the plants. The plants need the calcium as much as the fish do, and other minerals as well, you can look at the additives sold for reconstituting RO water or figure out the parts you need individually for less $.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside